I was thinking much the same thing, but the OP did specify compact. The 31/1.8 is a big, heavy horse of a lens. The same thing with an APS-C image circle should be much smaller. For that matter, my A35/2 is about half the size and 1/3 the weight.

But neither are what I consider a fast lens. Something in the 30mm range with an f/1.4 or faster maximum aperture would be nice.